The present invention and the problem upon which it is based will be explained in greater detail with reference to printing apparatuses that employ ink that is melted to a liquid state. More specifically, the ink is liquid at an elevated temperature and is generated by melting solid ink elements, such as toner pearls (i.e. so called “hot-melt” ink). In such a printing apparatus, in which the print-head uses hot-melt ink, the melted liquid ink is supplied from an ink reservoir to a drop-forming unit of the print-head. In such conventional print-head arrangements for hot-melt ink, the ink passes through and is heated in channels having a length of about 10 mm per print-head nozzle, just before the ink reaches the nozzle plate.
With more recent developments, modern drop-forming units employ micro-electro-mechanical systems (MEMS) provided on a chip, which can be supplied at high ink flow rates. In such MEMS chips, however, the ink channel is only about 1 mm long per nozzle, but the ink flow per nozzle remains unchanged. As a result, the heat transfer efficiency for heating the ink to the desired printing temperature in a MEMS print-head is very low. It therefore becomes necessary to deliver the ink to the chip in a MEMS print head with a much smaller temperature range than is the case for conventional hot-melt ink print-head arrangements to achieve the same temperature range and temperature gradient in the print-head nozzle. In contrast to conventional hot-melt ink print-heads, which have their own temperature sensor and heater, MEMS chips are not equipped with a temperature sensor or heater. As such, the temperature of the ink in the channel of the chip is far more difficult to control. The lack of temperature control in a MEMS chip means that a temperature range of the ink delivered to the chip has to be correspondingly narrower. In particular, for good ink drop quality (i.e. small drop volume variation) at the drop-forming unit, it is important that temperature variation over time and the gradients over the location of the ink in the MEMS chip are small.
In other words the uniformity of the temperature of ink entering a printhead having relatively short ink channels (e.g. a MEMS printhead) needs to be high in order to provide good drop quality which is also more or less constant in time.
The same holds for other ink types that need to be heated before being printed. Therefore, the present invention is not limited for use with hot-melt inks.
It is therefore an object of the present invention to provide a new and improved heating device for heating ink in an ink supply system as well as a new and improved ink supply system for supplying ink to a drop-forming unit in a print-head of a printing apparatus. The heating device according to the present invention enables high ink throughput and high temperature uniformity of the heated ink.